A treating method for metastatic cancers and details of a metastasis mechanism of a cancer are still unresolved problems. For example, although it is known a cancer cell metastasizes to another organ through a blood vessel, a detailed mechanism for this is unresolved. There is a possibility that a method for, for example, specifying the cancer cells in blood and counting the number of cancer cells can be used for prediction of convalescence of a cancer, evaluation of a treatment effect or the like (for example, see “The New England Journal of Medicine Vol. 351, pp. 781-791 (2004)”). Additionally, it is estimated that removal of cancer cells in blood allows a metastasis probability to be lowered. Regarding handling of such cancer cells in blood, for example, patent document 1: Japanese Patent Publication No. H6-61360 and document 2: Japanese Patent Publication No. H7-3419 disclose analysis of cells in blood.    Patent Document 1: Japanese Patent Publication No. H6-61360    Patent Document 2: Japanese Patent Publication No. H7-3419